The present invention relates to catalysts comprising a support optionally doped with oxides, hydroxides and hydrated oxides of the rare earths and of manganese onto which support Ru and Pd are applied and which are treated with alkaline compounds of the alkali or alkaline earth metals and are characterized in that they contain no halogen, and a process for preparing cycloaliphatic amines by hydrogenation of the corresponding aromatic precursors in the presence of the catalysts of the invention.
It is known that aromatic amines can be hydrogenated in various ways using various catalysts. However, the development of the relevant technology teaches that it is very difficult to solve all problems which occur, for example unsatisfactory conversion, lack of selectivity caused by hydrogenolysis or condensation to form relatively high molecular weight compounds, insufficient space velocities, activity and operating life of the catalysts, sufficiently well for industrial purposes. This is shown by the many proposals which have been made on this subject over the course of time.
When Ni and Co catalysts were first employed (GB 969 542), it was soon found that supported noble metals are better catalysts (DE-AS [German Published Specification] 1 106 319). However, deaminations and condensations to form polymers, the latter particularly in the hydrogenation of polyamines, were observed, so that the reactions were restricted to low temperatures and relatively long reaction times. In addition, it was observed that NH.sub.3 formed in the condensation impairs the activity of the noble metals (J. Org. Chem. 29, 3082-4 (1964), EP 324 984). On the other hand, addition of NH.sub.3 makes it possible to restrict the condensation to relatively high molecular weight products. Attempts have also been made to control the high exothermicity of the hydrogenation and the associated temperature increases and by-product formation by addition of solvents (NL 6 709 040, EP 67 058) or by hydrogenation at low pressure in the gas phase (FR 1 530 477).
The addition of basic compounds such as alkalis to Ru catalysts has been found to be advantageous. This enabled the formation of polymers to be limited (U.S. Pat. No. 3,636,108, EP 53 818).
It was later found that Ru in the presence of oxides or hydroxides of Cr and Mn gives catalysts which have a greater activity and selectivity and make it possible to use smaller amounts of catalyst and carry out a hydrogenation in the absence of NH.sub.3 (DE-AS [German Published Specification] 2 502 893, DE-AS [German Published Specification] 2 502 894). An additional treatment of such Ru--Cr--Mn catalysts with alkalis leads to longer operating lives and greater insensitivity of the catalysts to impurities in the starting material (EP 1 425).
Hydrogenation using Pd instead of Ru has been proposed repeatedly, but condensation occurs preferentially (dicyclohexylamine from aniline: EP 503 347) so that NH.sub.3 again has to be added to avoid it (EP 53 818).
The use of Ru and Pd together on a Cr--Mn-doped or alkalized support offers an advantage if long catalyst operating lives are to be achieved. However, condensation again occurs to a considerable degree and is reduced only at very low temperatures (EP 324 983, EP 324 984). Such catalysts are, for example, suitable for the flexible preparation of mixtures of cyclohexylamine and dicyclohexylamine.
A further advance in respect of selectivity and the formation of primary amines at high conversions is achieved by using Ru on RE-Mn-doped supports (RE=rare earths). For example, cyclohexylamine is obtained in a selectivity of up to 97% at 110.degree. C. (EP 351 661, EP 476 359).
However, all known catalysts and processes still had disadvantages in respect of conversion, selectivity to primary amines, operating life of the catalysts, necessity of using NH.sub.3, etc. A serious problem in catalyst beds for the continuous trickle-phase hydrogenation is the tendency of all Ru catalysts to catalyze deaminations and hydrogenolysis of the molecules right through to methane as the temperatures increase. Thus, the hydrogenation which is itself exothermic can, for example in the case of slight deviations from a given temperature, go over, first slowly then sometimes very rapidly, into the far more strongly exothermic methanization and lead to a situation which can no longer be controlled, even as far as explosions. For this reason, very comprehensive and reliable safety precautions have to be taken when using Ru catalysts. However, this calls into question the suitability of the Ru catalysts, which are very good per se, for industrial plants.
The problems which still occur even today despite the progress which has taken place are shown by EP 560 127 filed in 1992: The Ru--Pd catalysts on alkaline supports used here can hydrogenate aromatic amines at low pressure but they allow only small throughputs of from 0.03 to 0.05 g/ml of catalyst and hour, which demands large amounts of catalyst and large reactors; NH.sub.3 has to be added in large amounts and the temperatures are held in the vicinity of 160.degree. C. Despite this, hydrogenolysis still occurs, as can be recognized by the formation of benzene and cyclohexane, while the conversion continues to be incomplete; the selectivity leaves something to be desired and the operating life of the catalyst is significantly shorter than, for example, in EP 324 983. The commencement of deactivation of the catalyst is indicated by the slowly decreasing conversion.
It was accordingly an object of the invention to provide catalysts for the hydrogenation of aromatic amines to cycloaliphatic amines, which catalysts effect complete conversion at high space velocities, have high selectivity in respect of the formation of primary cycloaliphatic amines even without addition of NH.sub.3 and, in particular, no longer trigger hydrogenolysis and methanization of the substrates.
It has been found that these requirements can be met if use is made of catalysts which contain Ru and Pd and are preferably applied to alkalized supports or to RE-Mn-doped supports and are strictly halogen-free. The invention is surprising in that the influence of halide ions on the catalytically active Ru to cause hydrogenolysis and methanization as a result of the undesired over-activation described was not known. The invention is also surprising in that even small residual amounts of halide which remain in the catalyst after the preparation of such catalysts from halide-containing starting materials obviously cause this undesired property.